The burliest downhill bikes - ones with 8" of travel that look like motorcycles without engines - are aluminum. It's not the material as much as the engineering. That said, steel is more of a boutique thing, so you're going to get an aluminum bike. No worries.
To carry stuff, put it in your backpack. Ideally, get a Camelbak or a knock-off - with a water bladder and straw. Tools clank around too much on the frame, water bottles get full of dirt (assuming they don't fall off on the trail), etc.
At your price point you're frankly better off buying a BMX-like mountain bike - rigid, maybe even a 90's era mountain bike - and putting on the fattest tires you can fit. Shocks and suspensions at that price really suck, and 'oh it's OK if it breaks' may be easy to say on the internet, but hard to say when you're spitting up teeth. (I've fractured my skull, so I can say that. You don't want to do that.)
29ers are great. They're ideal for people over 6'. That said, they have strengths and weaknesses, so you have to decide what's more fun - 'flickable agility' or 'just rolls over stuff'.
I scored a 29er single speed - basically, a huge BMX bike - for $400 off Craigslist. It was $1k new. Be patient, deals are out there.
Last edited by schnee; 06-06-10 at 12:39 AM.